[turbofan] Don't spread global flag checks all over the compiler code.

src/compiler/source-position.cc

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/compiler/source-position.h"
 #include "src/compiler/graph.h"
 #include "src/compiler/node-aux-data.h"
 
 namespace v8 {
 namespace internal {
 namespace compiler {
 
 class SourcePositionTable::Decorator final : public GraphDecorator {
  public:
   explicit Decorator(SourcePositionTable* source_positions)
       : source_positions_(source_positions) {}
 
   void Decorate(Node* node, bool incomplete) final {
     DCHECK(!source_positions_->current_position_.IsInvalid());
     source_positions_->table_.Set(node, source_positions_->current_position_);
   }
 
  private:
   SourcePositionTable* source_positions_;
 };
 
 
 SourcePositionTable::SourcePositionTable(Graph* graph)
     : graph_(graph),
-      decorator_(NULL),
+      decorator_(nullptr),
       current_position_(SourcePosition::Invalid()),
       table_(graph->zone()) {}
 
 
 void SourcePositionTable::AddDecorator() {
-  DCHECK(decorator_ == NULL);
+  DCHECK_NULL(decorator_);
   decorator_ = new (graph_->zone()) Decorator(this);
   graph_->AddDecorator(decorator_);
 }
 
 
 void SourcePositionTable::RemoveDecorator() {
-  DCHECK(decorator_ != NULL);
+  DCHECK_NOT_NULL(decorator_);
   graph_->RemoveDecorator(decorator_);
-  decorator_ = NULL;
+  decorator_ = nullptr;
 }
 
 
 SourcePosition SourcePositionTable::GetSourcePosition(Node* node) const {
   return table_.Get(node);
 }
 
 
 void SourcePositionTable::Print(std::ostream& os) const {
   os << "{";
   bool needs_comma = false;
   for (auto i : table_) {
     SourcePosition pos = i.second;
     if (!pos.IsUnknown()) {
       if (needs_comma) {
         os << ",";
       }
       os << "\"" << i.first << "\""
          << ":" << pos.raw();
       needs_comma = true;
     }
   }
   os << "}";
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8